1. Field of the Invention
The present invention relates to a logarithmic amplification circuit, and more particularly, to a logarithmic amplification circuit incorporated in a semiconductor integrated circuit, which is capable of supplying output voltage corresponding to a difference between logarithmically amplified values of two input currents.
2. Description of the Background Art
FIG. 2 is a circuit diagram showing one example of a conventional logarithmic amplification circuit, and is, for example, described in a Japanese publication "Practical Technique of Operational Amplifier", pp. 150, issued Oct. 10, 1980. In the circuit of FIG. 2, an input terminal 3 is connected to a negative input end of a comparing circuit 1 through a resistor 2, and a positive input end of the comparing circuit 1 is grounded. An emitter-collector path of a transistor 5 having its base grounded is connected between the negative input end of the comparing circuit 1 and an output terminal 4. Now, if an input voltage of ei is applied to the input terminal 3, an output voltage e.sub.0 obtained at the output terminal 4 will be as follows: ##EQU1## wherein: q: electric charge of electron,
k: Boltzmann's constant PA1 T: absolute temperature, PA1 R: resistance value of the resistor 2, PA1 I.sub.S : saturation current of P/N junction
Accordingly, the circuit of FIG. 2 is able to logarithmically amplify the input voltage ei.
However, the logarithmic amplification circuit of FIG. 2 had a problem in that it could not obtain an output corresponding to a difference between logarithmically amplified values of two input signals. Although a logarithmic amplification circuit for two input signals may be employed as being provided with a differential amplification circuit of two inputs-one output at a prior stage to the logarithmic amplification circuit shown in FIG. 2, such a circuit has a problem in that it leads to increase of the number of elements and deterioration of characteristics.